1. Field of the Invention
The present invention relates to a voltage regulator (hereinafter referred to as V/R) capable of improving an overshoot characteristic of the V/R. The present invention also relates to an electronic device equipped with the voltage regulator.
2. Description of the Related Art
As shown in a circuit diagram of FIG. 3, a conventional V/R includes: a V/R control circuit composed of an error amplifier 13 that amplifies a differential voltage between a reference voltage Vref1 of a reference voltage circuit 10 and a voltage at a connection point of bleeder resistors 11 and 12 that divide a voltage (hereinafter referred to as an output voltage) Vout at an output terminal 6 of the V/R; and an output MOS transistor 14. The V/R operates according to a voltage (hereinafter referred to as VDD1) supplied from a voltage source 15. Assuming that an output voltage of the error amplifier 13 is given by Verr and a voltage at the connection point of the bleeder resistors 11 and 12 is given by Va, if Vref1>Va is established, Verr becomes lower. On the other hand, if Vref1<Va is established, Verr becomes higher.
If Verr becomes lower, because the output MOS transistor 14 is a P-ch MOS transistor in this case, a voltage between a gate and a source thereof becomes larger and an ON resistance becomes smaller, so that the V/R functions to raise the output voltage Vout. On the other hand, if Verr becomes higher, the V/R functions to increase the ON resistance of the output transistor 14 and to reduce the output voltage, thereby keeping the output voltage Vout at a fixed value (for example, see JP 04-195613 A (pages 1 to 3 and FIG. 2)).
Although not shown here, it is generally known that a phase compensating capacitor needs to be suitably added to the V/R if necessary.
Also, it is generally known that, for example, as shown in FIG. 4, the error amplifier 13 of the V/R includes: a current mirror circuit composed of a P-ch MOS transistor 16 and a P-ch MOS transistor 17; an input differential pair composed of an N-ch MOS transistor 18 and an N-ch MOS transistor 19; and a constant current circuit 20 into which a constant current 11 flows.
However, in the conventional V/R, an operating current of the error amplifier 13 is determined by a constant current circuit 20. Therefore, there arises the following problem. If a current flowing into the constant current circuit 20 is reduced in order to realize a V/R having low current consumption, when the power source is started, that is, when VDD1 is provided as a pulse signal, or when a load connected with the output terminal 6 of the V/R is suddenly reduced, the output voltage Vout largely tends to exhibit an overshoot characteristic. In other words, a power source start characteristic is sacrificed. On the other hand, if a current flowing into the constant current circuit 20 is increased in order to realize a V/R having an improved overshoot characteristic, the low current consumption characteristic is sacrificed.
When a battery is used as the power source, the low current consumption characteristic is required to lengthen a life of the battery. On the other hand, with regard to the overshoot characteristic of the output voltage Vout of the V/R, it is necessary to avoid a state in which the output voltage becomes equal to or larger than a withstanding voltage of an external element connected with the output terminal of the V/R.
When the overshoot characteristic of the V/R is improved, for the purpose of achieving a wide band of the error amplifier 13, it is basically unavoidable to increase the operating current of the error amplifier 13. However, when the battery is used as the power source, it is essential to obtain the low current consumption characteristic. Accordingly, in the current state, it is not acceptable to increase the current consumption of the V/R itself.